1. Field of Invention
The present invention relates to printed wiring board laminates and more particularly to substrate laminates suitable for use in the electronics industry. Specifically, the present invention relates to a method of bonding a multilayer wiring board to a dielectric-surfaced substrate, and the method of processing the substrate which results in improved dielectric strength, thermal conductivity, and bond strength between the multilayer circuit board and the substrate.
2. Description of Prior Art
Power processing circuits are typically characterized by high voltage, high current, and high power dissipation requirements. The trends toward increased power processing densities further increase the demands on circuit fabrication technology. Printed wiring boards represent a relatively low-cost method of fabrication. In a typical single layer board, a copper layer is laminated or electroplated to an organic board material. The organic board material prohibits high thermal dissipation due to its low thermal conductivity. Thermal resistance is usually specified for the area of a TO-220 semiconductor package. The TO-220 package represents a 0.25 in..sup.2 heat transfer area. Thermal resistance through this type of board is found to be around 11.degree. C/W, for a section 0.062 inch thick. Those skilled in the art have been successful in processing thinner sections using flexible polyimide. Typical resistance for a TO-220 package using a 2-mil thick polyimide layer is approximately 2.63.degree. C/W.
Insulated Metal Substrates (IMS) with a tri-layer construction have overcome some of the heat dissipation concerns. In this method of construction, the copper circuit layer is laminated to a metal substrate having high thermal conductivity. Metal substrates constructed of aluminum or copper are prevalent in the art. To provide the desired electrical insulation between the copper circuit layer and the metallic substrate, a dielectric insulator layer such as a thermally conductive epoxy or acrylic adhesive may be used. Circuit boards of this type, using a proprietary adhesive insulation layer can provide a thermal resistance of 0.31.degree. C/W when a 0.062 inch thick section is used (0.005 inch adhesive, 0.057 inch aluminum).
Circuit boards using dielectric ceramics often provide a better solution to the problem of heat dissipation, although at a much higher cost. It is widely known that copper is not readily deposited to an aluminum oxide layer with a bond strength that is acceptably high enough for use as a circuit board assembly. Because of this, there are four distinct types of methods for attaching copper circuit traces to a ceramic substrate. In the Thick Film method, a copper metal paste is applied to a fired ceramic substrate and re-fired to result in a copper circuit bonded to a dielectric ceramic. In the Thin Film method, the metal circuitry layer may be plated, sputtered, or vapor-deposited onto the pre-fired ceramic substrate. The Cofired method is similar to the Thick Film method but the metal paste and the ceramic slurry are fired simultaneously. The Direct Bond Copper (DBC) method is similar to the Cofired method, wherein an oxidized copper circuit pattern is placed onto the ceramic. The assembly is fired until the copper oxide reflows into the ceramic oxide. The most popular types of ceramics used are aluminum oxide (Al.sub.2 O.sub.3), beryllium oxide (BeO), and aluminum nitride (AlN). Because of the material property limitations of ceramics, i.e., brittleness, lack of machinability, etc., the ceramic is usually mounted to a metal substrate. The method of mounting is usually an adhesive, which re-introduces the problems associated with adhesives noted previously.
All of these types of basic power circuitry boards; organic epoxy, standard IMS, and ceramic substrate, suffer from the limitation that only one layer of copper circuitry can be used. Advances to these basic processes have allowed circuitry to be built on top of the base circuit layer, separated by a dielectric layer. Depending on the number of circuit layers, these multilayer circuit assemblies may allow much higher levels of circuit density. Still, each of these methods requires the use of an adhesive to attach the circuit to a substrate having the desired mechanical properties. In applications requiring enhanced thermal conductivity, the aforementioned united multilayer board structure is bonded by hot pressing to a heat sink structure using an adhesive. The desired adhesive must display good thermal conductivity and a high dielectric strength. The property of high dielectric strength is necessary to prevent the establishment of an electrically conductive path between the multilayer board structure and the heat sink structure.
Even though the as-manufactured laminate structure may exhibit a good tradeoff between thermal conductivity and dielectric strength, these properties may not be maintained during actual use. During thermal cycling in high humidity environments, the coefficients of thermal expansion of the various materials may cause a pumping action to occur. This pumping action can allow the introduction of water vapor into the adhesive. When the water vapor condenses and freezes, thus expanding, cracks may occur in the adhesive. As thermal cycling continues, these crack can increase in size and cause dielectric breakdown and a loss of bond strength leading to delamination, which in turn results in higher thermal resistance. In many cases, the introduction of water vapor will cause an electrogalvanic cell to become active. The corrosion cell is maintained by the water vapor in the adhesive, and the proximity of the aluminum substrate to the copper conductor layer and the voltage difference between the active copper conductor circuitry and the ground potential of the aluminum substrate.
Because of the well known limitations and compromises in thermal conductivity and dielectric strength of adhesives, those knowledgeable in the art may add a very thin (&lt;1-mil) dielectric polymer layer suspended within the interstitial adhesive layer, midway between the multilayer wiring board copper layer and the aluminum substrate. In this manner, the adhesive may be optimized for high thermal conductivity without regard for the necessary high dielectric strength--dielectric strength being supplied by the dielectric polymer layer. These methods, while largely eliminating electrogalvanic corrosion, still result in high thermal resistance because of the low thermal conductivity inherent in dielectric polymers.
Multilayer circuit boards may use several layers of epoxy laminate, polyimide, or other dielectric layers to join the intermediate layers of copper circuitry together. More recently multilayer wiring boards of a different type have been developed to meet the demand for multilayer wiring boards which can be made higher in wiring density and can be used in large computers. A multilayer wiring board of the new type may use polyimide resin as the interlaminar insulating material to support a plurality of wiring layers on a ceramic substrate. This type of multi-layer wiring board is produced by alternately repeating a process for forming a polyimide insulating layer and another process for forming a wiring layer. The former process includes applying a polyimide varnish to the substrate or a precedingly formed wiring layer, drying the varnish, forming via holes in the polyimide layer and then curing the polyimide. The latter process includes forming a wiring pattern by photolithography and then creating a wiring layer by vacuum evaporation or plating. Kimbara et al teaches a number of such techniques in U.S. Pat. No. 5,321,210.
However, as previously explained, thick ceramic substrates do not have good thermal conductivity and are known to be quite brittle, precluding their use as structural members. In applications where the use thick ceramics is mandated, these ceramics must be bonded to a heat sink structure material such as aluminum, with an adhesive displaying good thermal conductivity. As in the aforementioned fiber prepreg and flexible laminates, current technology adhesives lack the combination of good thermal conductivity and high dielectric strength that is necessary for high power devices. In addition, the ceramic substrates as a class are too expensive to use in economical commercial applications.
To transfer the heat from power components mounted on the top side of a multilayer circuit board to the bottom layer, copper-plated through-holes are used. The use of twenty, 0.028 inch diameter through-holes plated with 2-mils of copper can reduce the thermal resistance of an epoxy board from the aforementioned 11.degree. C/W down to 0.56.degree. C/W. To dissipate this heat, the multilayer board will still require attachment to a metallic substrate. Using the aforementioned proprietary adhesive, the thermal resistance becomes about 0.9.degree. C/W for a rigid epoxy resin board. Where then a thin (2-mil) section of polyimide is used instead of the epoxy, the thermal resistance can be reduced to 0.38.degree. C/W. Unfortunately, the largest thermal resistance, about 70% of the 0.38.degree. C/W, is within the adhesive layer, at 0.26.degree. C/W.
Clearly, in order to dissipate the heat from small, high power (&gt;10W) components, a better method of attaching the multilayer circuit to a metallic substrate is required.
Prior art techniques whereby the lower copper conductor of the multilayer wiring board structure is bonded to the aluminum substrate without a thermally insulative adhesive layer have been largely unsuccessful. Because one of the most widely used dielectrics, aluminum oxide (Al.sub.2 O.sub.3), forms readily on the surface of aluminum when exposed to oxygen, a number of techniques are known in the art for anodizing aluminum with this dielectric coating. Most of these techniques specifically pertain to producing lithographic grade aluminum printing plates. Most of these direct bonding techniques of the copper layer to the aluminum oxide have failed dielectrically and thermally because the oxide layer is porous. Depositing a copper conductor onto the aluminum oxide generally causes the copper atoms to diffuse into the aluminum oxide pores and diminish the dielectric strength of the aluminum oxide layer that is essential to the electronic circuit. Producing a thicker oxide layer on the aluminum substrate causes the oxide layer to become more porous. Therefore, the increase in dielectric strength does not increase linearly with increasing oxide layer thickness.
Prior art techniques whereby the thickness and porosity of the aluminum oxide layer are minimized by special baths and sealed pores, have lead to poor adhesion between the oxide layer and the copper conductor. This is thought to be due to the insufficient penetration and diffusion of the copper atoms into the sealed pores. This lack of molecular-level metal penetration into the aluminum oxide pores is thought to be due to vapor droplet size in vapor deposition and sputtering processes, and electrolyte wettability in electroplating operations.
Several techniques are known to those skilled in the art of producing tighter pores in aluminum oxide coatings on aluminum substrates. In these techniques, the tighter pores lead to a perception of higher dielectric strength only because the wettability of the electrolyte bath cannot penetrate the small pores of the aluminum oxide. Specifically, the values cited as dielectric strength across a somewhat porous material in the prior art cannot be considered accurate unless the method of testing and the current sensitivity are detailed. For example, many dielectric strength tests specify placing two needle-like probes against opposite faces of a dielectric material and increasing the voltage until current flows, thereby indicating a breakdown in dielectric properties. If a somewhat porous material is tested so that separate bodies of a low surface tension fluid are in contact with the aforementioned opposite faces of the dielectric material, the dielectric strength would be lower than in the previous test because the low surface tension fluid, having a high wetting action, would penetrate the pores to some degree and in effect, decrease the distance between the two probes. In another example, if each surface of the aforementioned somewhat porous dielectric material is metallically plated in an atom-by-atom process, the metal surface would penetrate much further into the dielectric material. Tested in this manner, the dielectric strength of this same material would be very much lower than in the needle-like probe test. In any of aforementioned test methods, instruments that register a dielectric breakdown strength of 50 .mu.A will indicate a lower dielectric strength than an identical instrument set at a monitoring level of 5 mA.
Dielectric strength testing is known in the art as a destructive test, whereby items that are tested in this manner may not be retested because of the destructive nature of the voltage and current path. Depending on the material used, when the voltage is increased to a point to overcome the resistance properties of the material, a current will flow along the path of least resistance. Repeated testing will yield a lower dielectric strength rating because a conductive path has now been established within the dielectric material, usually due to carbon tracking. One material which does not have this drawback is cellulose triacetate, which displays an instantaneous self-healing property. This material is also known to have very poor thermal conductivity which precludes its use in the field of thermally conductive circuit board laminates.
U.S. Pat. No. 4,481,083 to Ball et al teaches a method for producing an aluminum oxide layer on an aluminum film with a dielectric strength of up to 760 VDC for use in electrolytic capacitors. The method of dielectric strength testing is not specified, nor is there any mention of thermal conductivity, or bond strength to a copper deposition layer.
U.S. Pat. No. 4,894,126, and U.S. Pat. No. 4,898,651, both to Mahmoud describe a method for producing an aluminum oxide layer on an aluminum substrate for use as a substrate in an electronic circuit. The method of dielectric strength testing is not specified, nor is there any indication of thermal conductivity, or bond strength to a copper deposition layer.
It is widely known that most materials are not readily applied to an aluminum oxide layer with a sufficient bond strength, and there are techniques for increasing the adhesion strength.
U.S. Pat. No. 5,277,788 to Nitowski et al describes an aluminum substrate which has been twice-anodized and exposed to a third process which provides a leaving group of molecules to be reacted with an appropriate organic coating. This leaving group of molecules, while providing high adhesion, causes an unacceptably large decrease in thermal conductivity.
While the difficulty of bonding copper to aluminum oxide is a detriment when producing ceramic substrate electronic circuit boards, it is an advantage when producing the copper circuit traces that will be adhesively bonded to a rigid or flexible non-ceramic substrate. In fact, several techniques are known in the art for producing an easily peelable copper layer on aluminum oxide.
U.S. Pat. No. 4,431,710 to Lifshin et al describes a method for utilizing an oxidized aluminum carrier for the deposition and handling of a thin copper layer. Dielectric strength and thermal conductivity are not noted and are assumed to be unacceptable for use in aluminum substrate to multilayer wiring board laminates.
U.S. Pat. No. 5,322,975 to Nagy et al teaches the use of an aluminum oxide layer at least 1100 Angstroms thick to allow easier peeling of an electrodeposited copper layer. Dielectric strength and thermal conductivity are not noted and are assumed to be unacceptable for use in aluminum substrate to multilayer wiring board laminates.
None of the known prior art or literature describes a method for producing a multilayer circuit board which may be attached to a metallic substrate with the requirements of high thermal conductivity, high bond strength, and high dielectric strength. In fact, because of the nature of electron movement through an atomic lattice structure, the requirements of high dielectric strength and high thermal conductivity are most often in conflict.